1. Field of the Invention
This invention relates to the method of fabricating a semiconductor device in which the exposed end of at least one of the pn junctions is covered by a glass insulator and more particularly to the method of reducing the life time of the minority carriers in the semiconductor without increasing the leakage current in the reverse direction.
2. Description of the Prior Art
To enable semiconductor devices such as a diode, a transistor, a thyristor etc. to operate at high switching speeds, it is necessary to annihilate the excess minority carriers in the semiconductor devices as fast as possible. The velocity at which the excess minority carriers are annihilated is roughly known from the life time of the minority carriers. It is therefore essential for high speed operation to render the life time of the minority carriers as short as possible.
The conventional method of shortening the life time of the minority carriers as resorted to, for example, the use of heavy metal such as gold, which forms recombination centers in the semiconductor. The high temperature diffusion method is in most cases used to dope gold atoms into the semiconductor. According to this method, however, the distribution of the gold atoms in the semiconductor wafer is uneven and moreover the reproducibility and the controllability of the gold atom concentrations are hardly satisfactory. In addition, this method cannot be free from a fatal drawback that the leakage current increases when the PN junction is reversely biased.
Among the conventional methods of shortening the life time of minority carriers is also known a method which utilizes a fact that the lattice defects caused through the irradiation of semiconductor with radiation such as electron beams, gamma rays etc. serve as recombination centers, as disclosed in, for example, U.S. Pat. Nos. 3,809,582 and 3,888,701 wherein it is stated that the drawback of the method of shortening the life time due to the gold diffusion is eliminated and that the concentrations of the recombination centers have an excellent uniformity, reproducibility and controllability without increase in the leakage current in the reverse direction.
Another relevant prior art is the Japanese patent application laid-open No. 3271/72, "Method of improving the radiation-resistivity of a silicon transistor." According to this method, a silicon transistor having a SiO.sub.2 film, maintained at temperatures of 150.degree. to 450.degree. C., is irradiated with electron beams having an energy of lower than 150 KeV.
In order to improve the blocking voltage and the reliability of a semiconductor device, the material for surface passivation must be carefully chosen. Recently a resin-mold or plastic casing has been used to encapsulate a semiconductor device. In such a casing, glass insulator having an excellent resistivity to moisture and a high reverse-blocking voltage is preferably used as the surface passivation material for the semiconductor device.
However, the inventors' experiments have revealed that if a semiconductor device with the exposed ends of its pn junctions covered with glass layers is exposed to radiation at room temperatures, the leakage current in the reverse direction in the semiconductor device extremely increases. It was also found by the experiments that such an increase in the leakage current takes place manifestly particularly where glass insulator is used as surface passivation material, but that the increase in the leakage current hardly takes place where the passivation layer of SiO.sub.2 or resin is used, the SiO.sub.2 or resin passivation layer being conventionally known. Moreover, the experiments have made it clear that the increase in the reverse leakage current is dependent on the energy of the radiation source and its irradiation dosage and that the leakage current necessarily increases under such an irradiating condition that the life time of the minority carriers in the semiconductor device is substantially shortened. The increase in the leakage current must be prevented without fail since it is causative of the increase in the consumption of power by the semiconductor device and thermal runaway leading to the damage of the semiconductor device.